This invention relates to lead frames for molded rectangular flat packages having lead terminals extending outwardly through the sides of a resin body.
FIG. 2 shows a corner of a conventional lead frame 3 for a rectangular flat package which has a gate 5 for injecting a molding resin to provide a resin body 2 for the package. The corner of the resin body 2 has a nicked part 2a corresponding to the corner of the lead frame 3. Since the length of the nicked part 2a is small, the length r.sub.1 of each of two dam bars 1a and 1b of the lead frame adjacent to that corner can be made as large as that of each of the other dam bars 1c and 1d of the frame. The molding resin is injected into a cavity between an upper and a lower molding die through the gate 5 located at the bottom of the dam bar coupling portion 4 of the lead frame 3. Although some of the injected resin leaks through the boundary between the dies at the nicked part 2a of the corner of the molded portion 2 during the transfer molding, the dam bar coupling portion of the lead frame has a resin-blocking part 4a with a length r.sub.3 which extends along the nicked part and can be made at least equal to the length of the nicked part to enable the resin-stoppage part to prevent the leaking resin from spreading. With that arrangement, the length r.sub.1 of each of the dam bars 1a and 1b can be the same as the length r.sub.2 of the dam bars 2a and 2b. FIG. 2 shows four lead terminals 6 coupled together by the dam bars 1a, 1b, 1c and 1d and a central portion 7 of the lead frame.
However, because the length of the nicked part 2a is made small enough to make it possible to equalize the length r.sub.1 of each of the dam bars 1a and 1b to the length r.sub.2 of each of the other dam bars 1c and 1d, this conventional type of lead frame is unsatisfactory. This is because the short nicked part is likely to chip when the portion of the resin which leaked and has hardened on the nicked part at the gate 5 is removed after injection-molding the resin body 2. If the length of the nicked part 2a is increased to solve this problem, the length r.sub.3 of the resin-stoppage part 4a of the dam bar coupling portion 4, which extends along the nicked part, must be increased to be the same as that of the nicked part. As a result, the length r.sub.1 of each of the dam bars 1a and 1b at the corner of the molded portion 2 cannot be made as large as the length r.sub.2 of the other dam bars 1c and 1d as shown in FIG. 3. This will result in another problem since the dam bars 1a and 1b cannot be cut off by a dam bar punch 10 having a width corresponding to the length of each of the other dam bars 1c and 1d, because the punch would partially cut into the dam bar coupling portion 4 when it is operated.